This invention relates to electrophotographic processes and apparatus. In one of its more particular aspects this invention relates to a versatile multi-layered ion modulator and its use in an electrophotographic process which is capable of operating in both the positive and negative modes.
Electrophotographic reproduction techniques for making copies of graphic originals using photoconductive media are well known. Such processes generally call for applying a blanket eletrostatic charge to a photoconductor in the dark and then exposing the charged photoconductor to a pattern of light and shadow created by directing electromagnetic radiation onto a graphic original. The light-struck areas of the photoconductor are discharged leaving behind a latent electrostatic image corresponding to the original. A developed image is produced by applying an electroscopic powder to the latent electrostatic image and then fixing the image or transferring and fixing onto a suitable receiving medium such as plain paper.
This technique has been extended to foraminated structrues which are formed by applying a photoconductive layer to a conductive screen or similar apertured structure. Such structures function as ion modululators selectively passing a stream of ions through the apertures of the screen in a pattern corresponding to the graphic original to be reproduced.
The ion modulators which have been developed heretofore and are known in the prior art fall into several distinct classes:
The first is a two-layered screen or grid construction which is formed by applying a photoconductive layer onto an apertured metallic substrate. Such a structure is capable of accepting an electrostatic charge corresponding to a pattern of light and shadow created by electromagnetic radiation directed onto a graphic original. The operation and construction of such a device requires that the projection of ions through the screen occur simultaneously with the projection of the pattern of light and shadow. The simultaneity requirement is occasioned by the inability of such a system to retain or have any long "memory" in terms of the charge pattern imparted to the structure.
A second group of photoconductive screens has been adapted for use with charged material particles such as charged electroscopic powders but not gas ions. Such structures suffer from the deficiency that charged particles accumulate in those areas of structure which attract the particles. Ultimately, it is required that the screen be cleaned to physically remove the particles in order that the screen may be reused.
A three layer modulator is disclosed in co-pending applications Ser. Nos. 423,883 and 423,884 filed Dec. 12, 1973, of John D. Blades and Jerome E. Jackson assigned to the same assignee as this application. The three layered modulator described in the above mentioned application possesses a memory which makes it unnecessary to simultaneously image and project ions through the modulator.
While the prior art modulators have advanced the electrophotographic art there are disadvantages which need to be overcome in order to provide an ion modulator system which is sufficiently versatile so that copies can be made in both the positive and negative modes. It is also desirable to provide copies which have a high degree of contrast between image and background areas.